The present invention relates to a multi-layered optical filter film and production method thereof. More specifically, the present invention relates to thin film filter chips used as optical multiplexers and demultiplexers in optical communication systems.
Generally, an optical filter device is composed of a multi-layered thin film and utilizes interference of incident light in a multi-layered structure to selectively pass or transmit a particular wavelength range of the light. The conventional film filter device has a multi-layered structure of non-metal materials having different refractive indexes, formed on a glass substrate by vacuum evaporation. Each layer has a given thickness and a different refractive index set to form a filter having a specific filtering characteristic for a given wavelength.
Such film filter devices are generally utilized as multiplexers and demultiplexers in optical multi-communication systems. The multiplexer or demultiplexer is inserted in a branching point of a light guide network comprised of an optical fiber to effect spectral composition or separation. Therefore, the multiplexer and demultiplexer has a very small dimension and a very small film thickness on the order of several tens of micrometers to avoid light loss.
The conventional multiplexer and demultiplexer has a multi-layered film structure formed by superposing alternately optical dielectric materials having different refractive indexes on a glass substrate having a thickness of several tens of micrometers by means of regular vacuum evaporation. In production of such a filter device, after vacuum-evaporating the multi-layered film on a thick glass substrate having a thickness of several millimeters, the glass substrate is polished to reduce its thickness, and then the substrate is cut into chips of several millimeters square. Otherwise, the multi-layered film is deposited on thin glass substrate which is provisionally polished to a thickness of several tens of micrometers.